1. Field of the invention.
The present invention relates to a disposable absorbent article excellent in absorbency, and particularly to a sanitary napkin, paper diaper, toilet sheet or the like, characterized in that a non-woven fabric excellent in thermoprocessability is used as its surface material.
2. Description of the Related Art
The absorbent article according to the prior art, for example, a sanitary napkin or paper diaper, fundamentally comprises an absorbent layer made of cottony pulp, absorbent paper or the like, a leakproof layer attached to the bottom and the sides of the absorbent layer and a non-woven fabric covering the surface.
Recently, technology has rapidly advanced to develop new materials for an absorbent article, for example, a highly absorbent polymer or non-woven fabric made of synthetic fibers by a dry process, thus remarkably enhancing the absorbency of such an article. However, even an absorbent article constituted by materials each having excellent absorbency does not always exhibit sufficient performance, when practically used, which is apparent from the fact that the greatest complaint as yet of consumers against such an absorbent article is leakage from the crotch.
The greatest reason for the leakage is that the movement of the crotch of a wearer gives irregular stress to the absorbent article so as to cause separation of materials constituting the article or significant twist or wrinkle of the article. Among them, the non-woven fabric is in direct contact with the skin of a wearer and therefore is seriously stressed, so that the fabric frequently tends to be separated from a leakproof layer or an absorbent layer. Therefore, it has been expected that the fabric and these layers be unified by some method or other.
It is possible that a non-woven fabric is adhesive-bonded to a leakproof layer or an absorbent layer with a pressure-sensitive adhesive or a hot-melt adhesive to thereby unify both. However, this process necessitates complicated steps which result in prominently enhanced production cost.
A process which comprises melting a non-woven fabric by simple hot-pressing and bonding the melted fabric to the object, a so-called heat bonding process, if possible, allows high-speed production of the article without necessitating complicated steps and results in only slight enhancement in cost.
It can be understood from the above description that a non-woven fabric excellent in thermoprocessability is necessary for enhancing the leakproofness of an absorbent article, particularly under dynamic conditions. However, the non-woven fabric according to the prior art is quite insufficient in thermoprocessability. The problems of the non-woven fabric of the prior art can be broadly divided into the following three groups.
The first problem is that the meltability of a non-woven fabric is insufficient. This problem can further be resolved into two cases. In the first case, a non-woven fabric is made of fibers which can not be melted by heating, for example, rayon or acrylic fiber, so that the non-woven fabric can not be bonded to the object by melting at all. In the other case, a nonwoven fabric is made of a fiber which has a high melting point to exhibit insufficient fluidity even when melted, for example, polyester or nylon fiber, so that the processing temperature does not fall in a suitable range and the bond strength of the fabric to the object is low.
The second problem is that the melted non-woven fabric adheres to a heat sealer, so that the fabric is broken in the heat-bonded area or a processing machine is damaged. With the purpose of overcoming the above first problem, the blending of a low-melting fiber in a non-woven fabric has been carried out. However, in most cases, the fiber comprises components having the same melting point or components having melting points slightly different from each other like polyethylene/polypropylene conjugate fiber. Such fiber has a narrow melting point range, so that the whole of the fiber tends to be melted simultaneously in the thermoprocessing even by the slight deflection in processing temperature and the melted fiber is transferred to a sealer in a moment so that the processing machine is damaged and the fabric is broken in the heat-bonded area. The larger the content of such a low-melting fiber in a non-woven fabric, the more significant this problem. On the contrary, in the case wherein the content of a low-melting fiber in a non-woven fabric is low, even when the whole of the low-melting fiber is melted by heating, the melted fiber is prevented from transferring to a sealer to some extent in the temperature range wherein a high-melting fiber is maintained in a state of fiber, because the bonding or interlocking of the melted low-melting fiber to the high-melting fiber serves to prevent such transfer. However, the heat bonding effect is insufficient, because the content of a low-melting fiber is low. Additionally, the melted fiber still transfers to a sealer to some extent, so that neither breaking of a non-woven fabric in the heat-bonded area nor damage to a processing machine due to the melted fiber built up on the sealer, particularly by prolonged operation, can be substantially prevented.
The third problem is that it is difficult to produce a non-woven fabric having thermoprocessability and strength which are well-balanced with each other, while the strength is a basic physical property of a nonwoven fabric. Recently, a non-woven fabric produced by dry heat-bonding process and excellent in strength, touch and absorbency has been frequently used as a material for a disposable sanitary article such as sanitary napkin or paper diaper. The non-woven fabric produced by this process has a structure wherein fiber webs are fixed to each other by the heat-bonding between fibers themselves and therefore generally contains a low-melting fiber such as polyolefin at a high content. Such a non-woven fabric can not exhibit sufficient thermoprocessability, as described in the above description of the second problem. In order to improve the thermoprocessability of such a non-woven fabric a method which is reverse to that for improving the first problem has been carried out. That is to say, the blending of a high-melting fiber has been carried out to thereby improve the thermoprocessability to some extent.
However, a non-woven fabric prepared by this method has not only the problems indicated in the description of the second problem but also a disadvantage in that the bonding or interlocking between the two fibers is insufficient, so that the non-woven fabric exhibits a lowered strength and causes unnegligible fuzzing in some uses. Further, when such a non-woven fabric is produced by the thermal treatment under severe conditions for the purpose of enhancing the strength, the obtained fabric is rigid and exhibits lowered touch so that the excellent balance among the strength, touch and anti-fuzzing properties which is an advantage inherent to a non-woven fabric produced by dry heat-bonding process is detracted.
As described above, the non-woven fabric according to the prior art has disadvantages in that the thermoprocessing thereof is difficult and that it is very difficult to well-balance the thermoprocessability with the strength. Therefore, no absorbent article excellent in leakproofness has been produced as yet.